1. Field of the Invention
The present invention relates to a solution for integrating an industrial printing substrate transport system with digital printing units.
2. Description of the Related Art
More than a decade ago, multicolor inline screen printing systems began to make their appearance for printing multiple color large format graphics. They introduced improvements in print quality compared to a printing process using multiple single-color presses. The latter process suffered from substrate shrinkage and color registration problems between printing the different colors, particularly with thin paper and plastic substrates. Today, multicolor inline screen printing systems are highly automated and compete with offset printing for large format graphics. One of the benefits of multicolor presses is automated substrate handling. The majority of automated flatbed multicolor screen printing lines have an automated substrate handling system based on either gripper bars moving on a set of chains and pulling the printing sheet from one station to another (i.e., from one printing table to another) through the printing line, or moving platens wherein the entire platen or printing table, including the attached printing sheet, moves on a set of chains from one station to another through the printing line. The printing table is an important feature of the printing sheet transport system; it supports the printing sheet during transport through the printing line. In a screen print station, before the printing starts, the screen and the printing table holding the printing sheet are brought into a position facing each other at a distance called the off-contact distance. During printing, as the squeegee traverses along the print stroke, it pushes the screen against the printing sheet and presses the ink through the screen onto the printing sheet. The off-contact distance may range from “near contact” to as much as ⅜ inch or ½ inch, and depends on the size of the screen, the tension of the screen, the pressure of the squeegee on the screen, etc. Variations across the printing area of the off-contact distance are compensated by the pressure of the squeegee onto the screen so as to always ensure contact between the screen and the printing sheet during printing.
For digital non-impact printing technology, such as ink jet printing, it is known that the distance between the printing unit and the printing sheet is of major importance to enable correct operation of the printing technology. In ink jet technology, this distance is referred to as the throw-distance, and is typically in the range of 1 mm. Variations in throw-distance across the printing area are directly converted into variations in dot placement of printed pixels onto the printing sheet. Small variations in dot placement, especially if they are systematic, are known to be highly visible to the human eye. Therefore, the position of the printing table relative to the printing unit should be accurately controlled and consequently is often regarded as an important feature of the digital print station.
In low-end ink jet printers, the throw-distance is often fixed by design/manufacture and the range of printing substrates that can be used with these printers is often limited to paper like substrates (from a substrate thickness point of view). In multi-use ink jet printers, a wide range of printing substrates (at least from a substrate thickness point of view) can be printed on. These printers often include a feature allowing the printing unit and/or the printing table to be vertically adjusted to control the throw-distance. Published patent application U.S. 2004/0017456 to Obertegger et al. discloses an ink jet printer having three possible ways to adjust the throw-distance, i.e., (1) a vertical adjustment of a print head relative to a print head carriage, (2) a vertical adjustment of a complete print head carriage system relative to the printer frame, and (3) a vertical adjustment of the printing table relative to a base element that refers to the printer frame. In practice, the throw-distance is set once as a function of the substrate thickness before the printing starts and this setting is maintained during printing. In theory, the throw-distance may be adjusted continuously during printing if a distance sensor would be installed on the print head carriage to continuously monitor the distance between the print head and the printing substrate surface, as disclosed also in U.S. 2004/0017456 to Obertegger et al. In practice however, continuously activating the various elements of the throw-distance adjustment system would lead to the introduction of undesired vibrations and mechanical instability of those parts, such as the print head carriage or the printing table, of which it is the goal to position them at a fixed distance relative to each other. The one-off calibration of the throw-distance at the start of a print job has proven to work satisfactorily if the mechanical and dynamic properties of the moving and stationary elements of the printer that influence the throw-distance are such that the one-off calibration can be maintained throughout the print job. For example, the weight of the carriage may introduce bending of the guides for transversal movement of the carriage across the printing substrate, high accelerations of the carriage may introduce deformations and vibrations in the carriage itself, the guides, and support frame for the transversal movement of the carriage across the printing substrate, etc.
If digital printing technology is to evolve towards industrial applications, it needs to meet the requirements of more printing substrate flexibility, higher print throughput, and integration with existing industrial printing equipment. One way to advance industrial applicability of digital printing technology is the integration of digital printing with industrial screen printing. However, throw-distance control would be a problem for at least two reasons. Firstly, the printing table in industrial screen printing presses is considered a feature of the printing substrate transport system and not of the printing unit itself, making it more difficult to control throw-distance. Secondly, the size of the printing table and of the printing unit may be so large that it is a problem to maintain absolute or relative position accuracy of the printing components across the whole of the printing area during the printing process. For digital printing technology, position accuracy in the range of micrometers is required.
The inventors of the present application have discovered that it would be advantageous to have a printing system wherein the printing table can be an integral part of the digital printing unit during printing, and wherein the printing table can be an integral part of the printing substrate transport system during transport of the printing substrates to and from the printing table. The inventors of the present application have discovered that a printing system having this capability would be able to control throw-distance during printing and guarantee compatibility with industrial printing substrate transport systems.